feat(gcssource): add support for a GCS bucket source

[zachburg]

Based on https://github.com/google/nsscache/blob/main/nss_cache/sources/s3source.py and https://github.com/google/nsscache/blob/main/nss_cache/sources/httpsource.py.

I can't tell exactly what the file format is for files saved in S3, but I want the gcssource to consume "raw" passwd/group/shadow files like how the HTTP source works.

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Unit Test Results

    1 files      1 suites   2s :stopwatch: 303 tests 291 :heavy_check_mark: 11 :zzz: 1 :x:

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Results for commit 607b774a.

[jaqx0r]

Can you please format to appease yapf

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Unit Test Results

    1 files      1 suites   3s :stopwatch: 303 tests 292 :heavy_check_mark: 11 :zzz: 0 :x:

Results for commit 5c29ad4b.

[github-actions[bot]]

Unit Test Results

    1 files      1 suites   2s :stopwatch: 303 tests 292 :heavy_check_mark: 11 :zzz: 0 :x:

Results for commit 311a93a7.